The power converter portion of modern electronic equipment tends to be bulky and is often the limiting factor when attempting to miniaturize. In reducing power converter size, designers have turned to increased switching frequencies. Higher frequencies allow for smaller lighter inductive and capacitive energy storage devices, but also bring with them increased switching power losses.
The power dissipation of DC-DC power converters can be reduced by using zero-voltage switching (ZVS) techniques. Zero-voltage switching occurs when a power device begins conduction with a near zero-voltage across the device. Achieving zero-voltage switching over a large range of line voltages and loads is desirable to reduce electromagnetic interference. A practical example of zero-voltage switching occurs in the power devices of a phase-shifted full bridge converter under certain conditions.
U.S. Pat. No. 4,811,187, issued Mar. 7, 1989 to Nakajima et al discloses a magnetic-amplifier control scheme for a standard full bridge and is shown in FIG. 1. However, the magnetic-amplifier control will not work with a zero-volt switching full bridge, such as the ones shown in U.S. Pat. Nos. 4,860,189 and 5,132,889, both of which are hereby incorporated by reference. FIG. 2 shows the timing chart associated with FIG. 1 for operating the magnetic-amplifier control with a standard full bridge. Standard full bridge switching is controlled to achieve the desired output voltage V.sub.0.
In a standard full bridge with magnetic amplifier control, the saturable reactors are used to block a portion of the primary on-time resulting in a decrease of duty cycle to separately control the output V.sub.1. This decrease in duty cycle is shown in FIG. 2, between the time T0 and T1, where V.sub.C is the blocking voltage of the saturable reactor. The change in duty cycle can be seen by comparing V.sub.S with V.sub.F, where V.sub.S is the voltage induced in the secondary winding and V.sub.F is the voltage applied to the output inductor, between time T0 and T2. However, a freewheeling diode 32 is needed to provide an alternate path for output inductor current 31 during the time the primary 15 is not conducting current. It is not possible to have both forward diodes 25 and 27 conduct one-half of the inductor current as the main winding does, since one of the cores of the saturable reactors 47 or 49 was reset during the previous on-time and must block following this off-time. This results in the secondary current dropping to zero amperes and therefore no primary current will flow during the freewheeling interval of T2 through T4.
In a ZVS full bridge, it is necessary to have the primary current flowing during the primary clamped interval as disclosed, for example, in U.S. Pat. No. 5,132,889. Previous magnetic amplifiers do not allow the forward diodes and their corresponding saturable reactor to conduct until after the blocking interval. It would be advantageous for the forward diode to conduct as above and to also force the diode to continue conducting during both the primary clamped interval and during the first part of the next primary "on-time" while the opposite saturable reactor is blocking. It would be advantageous to have the zero-volt switching action to be improved or at least unaffected with a magnetic amplifier used on an auxiliary output.
It is an object of the present invention to provide one or more regulated auxiliary voltages from a single zero-volt full bridge power stage without degrading the zero-volt switching action.
It is a further object of the present invention to provide one or more regulated auxiliary voltages from a single zero-volt full bridge power stage without using a freewheeling diode in the output stage.